A catalyst may be defined as a substance which changes the rate of a chemical reaction without undergoing any net change itself. Many times only trace amounts of the catalytic material are sufficient to bring about manifold changes in the rate of chemical reaction. Although the phenomenon of catalysis was recognized in the mid 1850's, many challenges still exist in the development of efficient catalyst systems.
Over 40% of all petroleum products and 50% of all organic chemicals are dependent upon catalysts for their commercial production. The widespread use of catalysts in organic processes is largely due to their amenability to continuous processes with decreased labor costs. Oftentimes, higher purity and greater uniformity of product also result from the choice of catalysts.
Catalytic reactions are of two general types; homogenous catalysis and heterogenous. Catalysts that operate in the same phase as the reactants are known as homogeneous catalysts while catalysts that operate in a separate phase from the reactants are known as heterogeneous catalysts. In heterogeneous systems there is a distinct interface between catalyst and reactants. Catalysts speed up a reaction by lowering the activation energy.
There have been many attempts to prepare useful heterogeneous catalysts Bhadure, S., et. al., J. Chem. Soc., Dalton Tran. 1982, 445-450 prepared heterogeneous catalysts from divinylbenzene cross-linked polystyrene beads which were functionalized to support 2,4-pentanedionato complexes of certain metals. These metals included iron, copper, vanadium and zirconium. Cross-linked polystyrene beads were chloromethylated and treated with 2,4-pentanedione and sodium ethoxide in tetrahydrofuran. The functionalized beads were then heated with 2,4-pentanedionato metal complexes such that ligand exchange reactions resulted in polymer anchored metal complexes.
These catalysts suffer drawbacks when used as heterogeneous catalysts for liquid-phase transesterification reactions. The reason for the poor performance of these catalysts is that the metal leaches from the polymer into the reaction solution. In typical polymer-supported metal catalysts, the metal is attached to the polymer support by weak bonds which are ineffective at preventing the metal from leaching into the reaction solutions. Depletion of the metal in the heterogeneous catalyst results in decreased heterogeneous catalyst activity and may lead to undesired homogeneous catalysis.
The present invention solves the metal leaching problem by incorporating into the polymerization process a metal coordinated to one or more polydentate ligands to provide three or more chelate bonds to the metal, each of said polydentate ligands containing one or more pendant sites of ethylenic unsaturation.
The catalysts of this invention are found to readily tolerate 200-500 ppm levels of water in alcohol with no significant decrease in activity, in contrast to many other catalysts.